Excel rendering extension has been improved and now supports viewing in Excel 97 and 2000.

PDF rendering extension is more robust and has better performance.

Chart control provides more control over display styles.

References to external URLs (images and resources) from within a report are now supported.

Data caching behavior for report preview is now supported.

Newline in expressions is now supported.

The style of the HTML Viewer toolbar can now be modified through a style sheet.

New URL parameters offer more options for customizing report presentation at run time

Report Manager proxy persists authentication cookies so that they can be used by custom security extensions.

Hidden parameters are now supported.

Temporary snapshots can be compressed as well as stored on the file system.

Integrated security support for accessing report data sources can be disabled.

The Readme file is a good place to start to get the latest, in-depth info on the above improvements (okay, I know nobody reads the readme, but I'm a doc guy and I have to plug it, and it will seriously be worth your time if you are interested in any of the SP1 enhancements).